Power supply apparatus and display apparatus

ABSTRACT

A liquid crystal display apparatus is capable of reducing a power loss incurred by a power-supply section in a process of generating a power-supply voltage for driving a backlight section. In the configuration of the power-supply section, a main power-supply circuit and an inverter circuit (or a DC-DC converter) are connected to an input-voltage generation unit in parallel to each other, which is used for rectifying and smoothing the commercial alternative current power. The main power-supply circuit and the inverter circuit (or the DC-DC converter) each include an isolation transformer including a primary-side winding not isolated from the commercial alternative current power and a secondary winding provided on the secondary side. The direct current input voltage is supplied to the primary side to be subjected to a power conversion process to generate an output voltage on the secondary side of the isolation transformer. Thus, the number of power conversion process stages for supplying power to the backlight section connected to the inverter circuit (or the DC-DC converter) is reduced by 1. As a result, the power loss incurred by the power-supply section can be reduced to a value smaller than that of the conventional one.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a reissue application of U.S. Pat. No.7,764,022, issued on Jul. 27, 1010, which is a national phase entryunder 35 U.S.C. § 371 of International Publication No. WO 2005/112245,published Nov. 24, 2005, which claims the priority from JapaneseApplication No. P2004-145987, filed May 17, 2004, all of which arehereby incorporated herein by reference; more than one reissueapplication has been filed for the reissue of U.S. Pat. No. 7,764,022;the reissue applications are U.S. application Ser. No. 13/560,215 filedon Jul. 27, 2012 and U.S. application Ser. No. 14/989,388 filed on Jan.16, 2016.

TECHNICAL FIELD

The present invention relates to a power-supply apparatus for generatinga direct current power-supply voltage to be supplied to a displayapparatus and a predetermined load of the display apparatus as well asgenerating a power-supply voltage to be supplied to a backlight sectionof the display apparatus.

BACKGROUND ART

In a display apparatus of a non-self emitting type, a backlight sectionis employed as a light source for displaying a picture. An example ofthe display apparatus of a non-self emitting type is a liquid crystaldisplay apparatus.

The backlight section of such a liquid crystal display apparatus can bea cold cathode fluorescent tube or an LED (Light Emitting Diode).

In the case of a cold cathode fluorescent tube employed as a backlightsection, a power-supply section of the display apparatus includes aninverter circuit for generating an alternative current voltage used fordriving the backlight section.

As shown in FIG. 7, typically, such an inverter circuit inputs a directcurrent power supplied by a main power-supply circuit employed in aliquid crystal display apparatus, and generates an alternative currentvoltage.

In the liquid crystal display apparatus shown in the figure, first ofall, a rectification/smoothing circuit 101 inputs a commercialalternative current power-supply AC and generates a direct currentvoltage. Then, a DC-DC converter connected to the rear stage of therectification/smoothing circuit 101 to serve as a main power-supplycircuit 102 carries out a DC-DC power conversion process on the directcurrent voltage generated by the rectification/smoothing circuit 101 togenerate a direct current power-supply voltage at a stabilizedpredetermined level. Typically, the main power-supply circuit 102employs an isolation transformer serving as a direct current isolatorbetween the primary and secondary sides. That is to say, the primaryside, which is the commercial alternative current power-supply side,inputs a direct current voltage whereas the secondary side outputs adirect current power-supply voltage.

As shown in the figure, the direct current power-supply voltage outputby the secondary side of the main power-supply circuit 102 is suppliedto a load 103, which is driven by the direct current power-supplyvoltage to operate. In addition, also as shown in the figure, the directcurrent power-supply voltage output by the secondary side of the mainpower-supply circuit 102 branches, being supplied to an inverter circuit104.

The inverter circuit 104 carries out a DC-AC power conversion process onthe received direct current power-supply voltage to generate analternative current voltage, which is supplied to a backlight section105. The backlight section 105 is driven by the alternative currentvoltage to emit light.

In this case, the main power-supply circuit 102 is provided with aswitching converter on the primary side and a rectification/smoothingcircuit on the secondary side. In this configuration, a switching outputobtained on the primary side is rectified and smoothed on the secondaryside to generate a direct current voltage serving as the power-supplyvoltage. Thus, as shown in the figure, the inverter circuit 104 receivesthe direct current power-supply generated by the secondary side of themain power-supply circuit 102.

Then, as described above, the inverter circuit 104 carries out a DC-ACpower conversion process on the received direct current power-supply togenerate an alternative current voltage, which is supplied to thebacklight section 105 as a driving voltage.

On the other hand, FIG. 8 is a diagram showing the configuration of aliquid crystal display apparatus having a backlight section composed ofLEDs. It is to be noted that sections shown in FIG. 8 as sectionsidentical with their respective counterparts explained earlier byreferring to FIG. 7 are denoted by the same reference numerals as thecounterparts.

In the case of a backlight section 110 composed of LEDs as shown in thefigure, chopper regulators 109 are provided on the secondary side as acircuit for driving the backlight section 110. In the case of thetypical configuration shown in FIG. 8, a plurality of chopper regulators109a, 109b and 109c are connected in parallel to each other to aplurality of LEDs, which form the backlight section 110.

To be more specific, the chopper regulators 109a, 109b and 109c are eachconnected to a circuit including a plurality of LEDs connected to eachother in series. The chopper regulators 109 receive a direct currentvoltage from the secondary side of the main power-supply circuit 102 andcarries out a DC-DC power conversion process on the direct currentvoltage. Then, a direct current voltage obtained as a result of theDC-DC power conversion process is stabilized in a voltage stabilizationprocess according to a result of detecting the level of a currentflowing through each of the LEDs. The stabilized voltage is applied tothe LEDs as a driving voltage to emit light from each of the LEDs.

The chopper regulators 109 are connected in parallel to each other inthis configuration to keep up with, for example, a case in which thenumber of LEDS is relatively large to form a big-size screen of theliquid crystal display apparatus and a case requiring a relatively highlevel as a direct current level for producing necessary high luminance.That is to say, if only one chopper regulator 109 is used for driving aplurality of LED series connection circuits in a configuration includinga large number of LEDs to be driven and a configuration requiring alarge current as described above, the size of the circuit of the singlechopper regulator 109 itself is big and, in order to solve this problem,a plurality of chopper regulators 109 is connected in parallel to eachother.

If a backlight section 110 including LEDs as described above is used,the chopper regulators 109 receive a direct current voltage from themain power-supply circuit 102 and carries out a DC-DC power conversionprocess on the direct current voltage. Then, a direct current voltageobtained as a result of the DC-DC power conversion process is used as adirect current power-supply of the backlight section 110.

It is to be noted that Japanese Patent Laid-open No. Hei 2-79182discloses a technology relating to an inverter circuit provided for acase in which a fluorescent tube is used as a light source of a displayapparatus.

In addition, Japanese Patent Laid-open No. 2002-244103 discloses atechnology relating to a chopper regulator provided for a case in whichLEDs are used as a light source.

By the way, as shown in FIG. 7 previously, the inverter circuit 104 isprovided at the stage behind the main power-supply circuit 102. Thus, apower supplied to the inverter circuit 104 is a result of a powerconversion process carried out in the main power-supply circuit 102.Then, in order to generate an alternative current voltage for drivingthe backlight section 105, the inverter circuit 104 again carries out apower conversion process.

That is to say, in the conventional configuration shown in FIG. 7, inorder to drive the backlight section 105, two power conversion processesare carried out in the main power-supply circuit 102 and the invertercircuit 104 respectively.

Also in the case of the configuration shown in FIG. 8, in order to drivethe backlight section 110, two DC-DC power conversion processes arecarried out in the main power-supply circuit 102 and the chopperregulator 109 respectively.

By carrying out a plurality of power conversion processes as describedabove, the power conversion efficiency decreases, raising the powerloss.

Particularly, in recent years, a technological revolution in the liquidcrystal display field increases the size of the display screenincreasing thereby the power consumption for driving the backlight sothat the power consumption of the set as a whole rises. For a screensize of 40 inches, for example, the power consumption of the set as awhole is about 250 W for some cases. In the case of display apparatusmanufactured in recent years as apparatus with large screens, the powerloss reaches a relatively high level.

In addition, in such cases, with the display screen increased and thepower consumptions of the inverter circuit 104 and the chopper regulator109 rising, in consequence, it is necessary to keep up with the largepower of the main power-supply circuit 102. That is to say, since theinverter circuit 104 and the chopper regulator 109 are provided at thestages behind the main power-supply circuit 102, as the powerconsumptions of the inverter circuit 104 and the chopper regulator 109increase, the power of the main power-supply circuit 102 also rises byan increase that should be kept up with.

Thus, in the conventional configurations shown in FIGS. 7 and 8, as thesize of the display screen increases, the size of the main power-supplycircuit 102 also rises so that the cost to manufacture the circuit ofthe main power-supply circuit 102 becomes higher as well.

In addition, by letting the large power of the main power-supply circuit102 be kept up with as described above, the amount of heat dissipateddue to an actual power loss increases. In order to keep up with theincreasing amount of dissipated heat, it is necessary to reserve a spacelarge enough to serve as a countermeasure of the dissipated heat or toset a countermeasure by providing a cooling fan.

If a space is provided as a countermeasure of dissipated heat, however,the space certainly entails a large size of the apparatus. In addition,if a cooling fan is provided, the operation sound of the fan will serveas a source of discomfort suffered by the user.

DISCLOSURE OF INVENTION

In order to solve the problems described above, the present inventionprovides a power-supply apparatus having the following configuration.

In the first place, the power-supply apparatus has:

an input voltage generation section for generating a direct currentinput voltage from an input alternative current voltage; and

a first power conversion section including a primary side for receivingthe direct current input voltage as well as a secondary side isolatedfrom the primary side and used for generating a direct currentpower-supply voltage to be supplied to a predetermined load as a resultof a DC-DC power conversion process carried out.

In the second place, the power-supply apparatus includes a second powerconversion section including a primary side for receiving the directcurrent input voltage as well as a secondary side isolated from theprimary side and used for generating an alternative current voltage tobe supplied to a backlight section as a result of a power conversionprocess based on DC-AC conversion of the direct current input voltage.

In addition, the power-supply apparatus further includes a displaysection for displaying a picture by using the backlight section.

In accordance with the above configuration of the present invention, thesecond power conversion section operates by directly inputting thedirect current input voltage generated by the input-voltage generationsection instead of inputting the direct current output voltage generatedby the first power conversion means.

That is to say, the present invention does not adopt a circuitconfiguration in which a power conversion process is carried out aplurality of times.

As described above, in accordance with the present invention, theconfiguration for generating a voltage used for driving the backlightsection of the display apparatus does not include a circuit for carryingout a power conversion process a plurality of times. Thus, the powerloss of the power-supply apparatus can be reduced to a value smallerthan the conventional configuration.

In addition, in accordance with the present invention, the first powerconversion section and the second power conversion section are notconnected to each other in series. Instead, they are connected to eachother to form a parallel circuit receiving the direct current inputvoltage. Thus, the power to be consumed by the first power conversionsection is independent of the power consumption of the second powerconversion section. As a result, even if the power consumption in a loadconnected to the second power conversion section rises, it is no longernecessary to increase the capacity of the first power conversionsection.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a simplified configuration of a power-supplyapparatus employed in a liquid crystal display apparatus according to afirst embodiment of the present invention;

FIG. 2 is a circuit diagram showing a typical configuration of arectification/smoothing circuit employed in the power-supply apparatusaccording to the first embodiment;

FIG. 3 is a circuit diagram showing the configuration of an inverteremployed in the power-supply apparatus according to the firstembodiment;

FIG. 4 is a circuit diagram showing another simplified configuration ofthe power-supply apparatus according to the first embodiment;

FIG. 5 is a circuit diagram showing a typical configuration of a PFCconverter circuit employed in the power-supply apparatus according tothe first embodiment;

FIG. 6 is a circuit diagram showing a configuration of a power-supplyapparatus employed in a liquid crystal display apparatus according to asecond embodiment of the present invention;

FIG. 7 is a block diagram showing a simplified configuration of apower-supply apparatus employed in the conventional liquid crystaldisplay apparatus employing a backlight section based on fluorescenttubes; and

FIG. 8 is a block diagram showing a simplified configuration of apower-supply apparatus employed in the conventional liquid crystaldisplay apparatus employing a backlight section based on LEDs.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments implementing the present invention are describedbelow. In the following description, the preferred embodiments are eachreferred to simply as an embodiment.

FIG. 1 is a block diagram showing a configuration including a simplifiedconfiguration of a power-supply apparatus 10 serving as a power supplysupplying power to a display apparatus 20, for example, which is aliquid crystal display apparatus according to a first embodiment.

First of all, the power-supply apparatus 10 according to the embodimentfunctions as a power-supply section of a liquid crystal displayapparatus. Power-supply voltages generated by the power-supply apparatus10 are supplied to a load 3 corresponding to a variety of circuits eachdriven to operate by a direct current power supply and a backlightsection 5 driven by an alternative current voltage. The backlightsection 5 radiates light to a panel back face of a liquid crystaldisplay unit functioning as a display section 6, causing the liquidcrystal display section to display a picture.

In the configuration shown in FIG. 1, a rectification/smoothing section1 rectifies and smoothes an input commercial alternative currentpower-supply AC to generate a direct current input voltage Ei.

The rectification/smoothing section 1 has a typical configuration shownin FIG. 2. The configuration shown in FIG. 2 includes a bridgerectification circuit Di and a smoothing capacitor C1. The bridgerectification circuit Di has 4 rectification diodes D1 to D4. Thesmoothing capacitor C1 is a capacitor for smoothing a rectified outputgenerated by the bridge rectification circuit Di.

As shown in the figure, a plus input terminal of the bridgerectification circuit Di is connected to a plus line of the commercialalternative current power-supply AC. On the other hand, a plus outputterminal of the bridge rectification circuit Di is connected to the plusterminal of the smoothing capacitor C1. The minus terminal of thesmoothing capacitor C1 is connected to the earth of the primary side. Aminus input terminal of the bridge rectification circuit Di is alsoconnected to the earth of the primary side. A minus output terminal ofthe bridge rectification circuit Di is connected to a minus line of thecommercial alternative current power-supply AC.

In the rectification/smoothing section 1 having the configurationdescribed above, during a half-cycle period of a positive polarity ofthe input voltage of the commercial alternative current power-supply AC,the rectification diodes D1 and D3 are in an electrically conductivestate, electrically charging their rectified output into the smoothingcapacitor C1. During a half-cycle period of a negative polarity of theinput voltage of the commercial alternative current power-supply AC, onthe other hand, the rectification diodes D2 and D4 are in anelectrically conductive state, electrically charging their rectifiedoutput into the smoothing capacitor C1.

That is to say, rectified outputs are electrically charged into thesmoothing capacitor C1 during both the half-cycle period of the positivepolarity and the half-cycle period of the negative polarity of the inputvoltage of the commercial alternative current power-supply AC to carryout rectification and smoothing operations based on full-waverectification. As a result of such rectification and smoothingoperations, a direct current input voltage Ei is obtained at theterminals of the smoothing capacitor C1 at a direct current level equalto the amplitude of the commercial alternative current power-supply AC.This configuration can be said to be a configuration for generating thedirect current input voltage Ei by adoption of the so-called capacitorinput method.

It is to be noted that the configuration of the rectification/smoothingsection 1 is not limited to the one shown in FIG. 2. That is to say,another configuration can also be adopted as a configuration forimplementing the capacitor input method. For example, the configurationcan be implemented as a multiplied-voltage rectification/smoothingcircuit.

In the case of this embodiment, a main power-supply circuit 2 and aninverter circuit 4 are connected in parallel to therectification/smoothing section 1 as shown in FIG. 1.

The main power-supply circuit 2 includes an isolation transformerbetween the side of the commercial alternative current power-supply ACand the side of the load 3. The main power-supply circuit 2 adopts theconfiguration of the so-called switching converter including a switchingdevice on the primary side of the isolation transformer and arectification/smoothing circuit on the secondary side of the isolationtransformer. The switching device employed in the main power-supplycircuit 2 serving as the switching converter switches the direct currentinput voltage Ei received from the rectification/smoothing section 1. Aresult of the switching operation excites an output on the secondaryside of the isolation transformer. The rectification/smoothing circuiton the secondary side of the isolation transformer then rectifies andsmoothes the output excited on the secondary side of the isolationtransformer to generate a direct current voltage, which is supplied tothe load 3 shown in the figure as an operating power-supply (a directcurrent power-supply voltage).

The inverter circuit 4 also receives the direct current input voltagefrom the rectification/smoothing section 1 to generate an alternativecurrent voltage for driving the backlight section 5.

To put it in detail, the inverter circuit 4 has the followingconfiguration. Not isolated from the commercial alternative currentpower-supply AC in a direct current isolation way, the primary side ofthe inverter circuit 4 receives the direct current input voltage Eigenerated by the rectification/smoothing section 1. The direct currentinput voltage Ei received by the primary side of the inverter circuit 4in this way is then subjected to a DC→AC power conversion process togenerate an alternative current voltage on the secondary side, which isisolated from the commercial alternative current power-supply AC in adirect current isolation way.

A typical internal configuration of the inverter circuit 4 is shown inFIG. 3.

The configuration shown in FIG. 3 is a separate excitation configurationin which switching devices Q1 and Q2 are driven in accordance withcontrol of a control/driving circuit 4a, which is shown in the figure,to generate an alternative current voltage for driving the backlightsection 5. The backlight section 5 driven by the inverter circuit 4typically includes 4 fluorescent tubes 14, which are denoted byreference numerals 14a to 14d respectively as shown in the figure.

First of all, the direct current input voltage Ei generated by therectification/smoothing section 1 shown in FIG. 1 is applied betweenterminals t1 and t2 of the configuration shown in FIG. 3.

The terminal t1 is connected to the drain of a MOS-FET serving as theswitching device Q1. The source of the switching device Q1 is connectedto the drain of another MOS-FET serving as the switching device Q2.

The source of the switching device Q2 is connected to the terminal t2.

The gates of the switching devices Q1 and Q2 receives control signalsfrom the control/driving circuit 4a.

The control/driving circuit 4a is a programmed IC (Integrated Circuit)for executing control to turn on and off the switching devices Q1 and Q2alternately.

The junction point between the source of the switching device Q1 and theswitching output point of the switching device Q2 is connected to aterminal of a primary winding Na1 of a transformer T1 shown in thefigure and a terminal of a primary winding Nb1 of a transformer T2,respectively. The other terminal of the primary winding Na1 is connectedto the terminal t2 through a capacitor C2. In the same way, the otherterminal of the primary winding Nb1 is also connected to the terminal t2also through the capacitor C2.

In this configuration, the primary winding Na1 of the transformer T1 andthe primary winding Nb1 of the transformer T2 are both not isolated fromthe commercial alternative current power-supply AC. That is to say, inthis configuration, as is obvious from the fact that the direct currentinput voltage Ei is applied between terminals t1 and t2, the front stagestarting with the primary windings Na1 and Nb1 employed in the invertercircuit 4 exists on the primary side not isolated from the commercialalternative current power-supply AC in a direct current isolationmanner.

Thus, with the inverter circuit 4 having the configuration describedabove, the backlight section 5 provided at the rear stage serves as aload on the secondary side isolated from the primary side in a directcurrent isolation manner by the transformers T1 and T2 for assuring thedirect current isolation state. Thus, it is necessary to preserve anenough direct current isolation state between the primary and secondarystates by, for example, allocating a sufficient gap between the primarywinding Na1 and secondary winding Na2 of the transformer T1 as well as asufficient gap between the primary winding Nb1 and secondary winding Nb2of the transformer T2 in the transformers T1 and T2 respectively.

As shown in the figure, one terminal of the secondary winding Na2 of thetransformer T1 is connected to capacitors CC1 and CC2 forming a parallelcircuit with respect to the terminal as a circuit for limiting a currentflowing through the secondary winding Na2. The capacitor CC1 isconnected to one terminal of the fluorescent tube 14a and the capacitorCC2 is connected to one terminal of the fluorescent tube 14b.

The other terminal of the fluorescent tube 14a and the other terminal ofthe fluorescent tube 14b are connected to the other terminal of thesecondary winding Na2.

By the same token, as shown in the figure, one terminal of the secondarywinding Nb2 of the transformer T2 is connected to capacitors CC3 and CC4forming a parallel circuit. The capacitor CC3 is connected to oneterminal of the fluorescent tube 14c and the capacitor CC4 is connectedto one terminal of the fluorescent tube 14d. The other terminal of thefluorescent tube 14c and the other terminal of the fluorescent tube 14dare connected to the other terminal of the secondary winding Nb2.

A feedback circuit 4b receives a tube voltage detected by a detectioncircuit 4c shown in the figure as a voltage of the fluorescent tube 14dand rectifies the peaks of the voltage. Then, the feedback circuit 4bsupplies a voltage obtained as a result of rectification to thecontrol/driving circuit 4a. On the basis of this voltage representing anadjusted optical signal, the control/driving circuit 4a controls thequantity of light emitted by the fluorescent tubes 14a to 14d to aconstant value.

It is to be noted that the feedback circuit 4b isolates the primary andsecondary sides from each other for example by using a photo coupler.

In the inverter circuit 4 having the configuration described above, thedirect current input voltage Ei applied between the terminals t1 and t2is switched on and off by the switching devices Q1 and Q2, which turn onand off alternately in accordance with control executed by thecontrol/driving circuit 4. A voltage output as a result of the switchingoperations is supplied to the primary windings Na1 and Nb1 employed inthe transformers T1 and T2 respectively.

By supplying the switching output to the primary windings Na1 and Nb1 asdescribed above, alternative current voltages are excited along thesecondary windings Na2 and Nb2 at a high level according to the windingratio of the primary windings Na1 and Nb1. The alternative currentvoltages excited along the secondary windings Na2 and Nb2 cause currentsto flow through the fluorescent tubes 14a to 14d. As a result, light isemitted from each of the fluorescent tubes 14a to 14d.

It is to be noted that, while the inverter circuit 4 adopts the separateexcitation method, a self-excitation technique can also be taken.

As described above, in the first embodiment, the main power-supplycircuit 2 and the inverter circuit 4 are connected in parallel to eachother to the rectification/smoothing section 1. Thus, in such aconfiguration, an alternative current voltage for driving the backlightsection 5 can be obtained not through the main power-supply circuit 2.As a result, the main power-supply circuit 2 does not incur a power lossdue to generation of the alternative current voltage for driving thebacklight section 5.

In addition, since the alternative current voltage for driving thebacklight section 5 can be obtained by carrying out a power conversionprocess only once in the inverter circuit 4, the power loss incurred inthe power-supply apparatus can be reduced to a small value in comparisonwith the conventional configuration shown in FIG. 7.

The reduction of the power loss is explained in terms of expressions asfollows.

First of all, let us assume that notation η1 denotes the powerconversion efficiency of the main power-supply circuit, notation η2denotes the power conversion efficiency of the inverter circuit,notation P1 denotes the load power of all loads other than the backlightsection 5 and notation P2 denotes the load power of the backlightsection 5. In this case, the input power of the conventionalconfiguration shown in FIG. 7 is expressed as follows:(1/η1)P1+(1/η1η2)P2

On the other hand, the input power of the configuration according to theembodiment shown in FIG. 1 is expressed as follows:(1/η1)P1+(1/η2)P2where the quantities of notation η1 denoting the power conversionefficiency of the main power-supply circuit 2 and notation η2 denotingthe power conversion efficiency of the inverter circuit are the same asthose in the configuration shown in FIG. 6.

That is to say, in the configuration including an inverter circuitprovided at the stage behind the main power-supply circuit as shown inFIG. 7, the power conversion efficiency along a path for obtaining thealternative current voltage at the output of the inverter circuit is aproduct of the power conversion efficiency of the main power-supplycircuit and the power conversion efficiency of the inverter circuit.Thus, the power conversion efficiency along the path decreasessubstantially due to the multiplication.

In the case of the embodiment, on the other hand, the power conversionefficiency along the path for obtaining the alternative current voltageis dependent only on the inverter circuit. Thus, the power conversionefficiency can be maintained at a value higher than that of theconventional power-supply circuit shown in FIG. 7. That is to say, thepower loss is small in comparison with the conventional power-supplycircuit.

Since the power conversion efficiency along a path for obtaining thealternative current voltage used for driving the backlight section canbe maintained at a value higher than that of the conventionalpower-supply circuit, the power loss caused by an increased load powerP2 of the backlight section due to an increased size of the displayscreen for example can be suppressed to a quantity smaller than that ofthe conventional power-supply apparatus.

That is to say, in this case, the difference in input power between theconfiguration shown in FIG. 7 and the power-supply apparatus 10according to the embodiment is expressed as follows:(1/η1η2−1)P2

As is obvious from the above expression, the greater the load power P2of the backlight section, the larger the difference in input powerbetween the conventional configuration and the power-supply apparatus 10according to the embodiment.

It is thus clear that, with the power-supply apparatus 10 according tothe embodiment, the larger the size of the screen display and, hence,the larger the power consumption of the inverter circuit 4, the greaterthe power-loss reduction effect as compared with the conventionalconfiguration.

In addition, as described above, since an alternative current voltagefor driving the backlight section 5 can be obtained not through the mainpower-supply circuit 2, it is no longer necessary to keep up with arising power of the main power-supply circuit 2 even if the size of thedisplay screen is increased. Thus, the amount of heat dissipated in themain power-supply circuit 2 due to an increased display screen in sizedoes not rise. As a result, it is no longer necessary to allocate asufficient space as a countermeasure to cope with dissipated heat as isthe case with the conventional power-supply apparatus. Accordingly, thedownsizing of the display apparatus is possible.

In addition, it is no longer necessary to provide a cooling fan forcoping with dissipated heat. It is thus possible to get rid of theoperation sound of the fan as a source of discomfort suffered by theuser.

On top of that, since the main power-supply circuit 2 no longer needs tosupply power to the backlight section 5, power-supply specifications ofthe main power-supply circuit 2 need to be dependent on only conditionsof the load 3. Thus, the design of the main power-supply circuit 2 canbe standardized with ease.

In the case of the conventional configuration shown in FIG. 7, on theother hand, the power-supply specifications of the main power-supplycircuit 2 are dependent also on, among others, the type of the backlightsection 105 (the type of the display panel). This is because thepower-supply specifications of the main power-supply circuit 2 aredependent on specifications of the inverter circuit and thespecifications of the inverter circuit need to be modified in accordancewith the type of the backlight section 105 (the type of the displaypanel). Thus, the design of the main power-supply circuit 2 can not bestandardized with ease.

In the conventional configuration, the main power-supply circuit and theinverter circuit cannot be connected to form a parallel circuit as isthe case with the embodiment described above due to the followingreasons.

In the field of the conventional liquid crystal display apparatus,display apparatus having a small screen with a size in the range 15 to17 inches are the majority. Thus, the power consumption of the invertercircuit is relatively small. Accordingly, in the case of theconventional configuration, the power loss incurred in a process ofgenerating an alternative current voltage for driving the backlightsection can be suppressed to a comparatively low level. For thesereasons, the conventional configuration including an unisolated inverterinputting power from the main power-supply circuit is rather preferredto offer merits due to the fact that that this configuration does notraise problems of an increasing cost and a rising circuit space.

The idea of the present invention has been adopted because, in the firstplace, the screen of the liquid crystal display apparatus has beenincreasing in recent years, causing the power consumption of thebacklight section to rise.

That is to say, in recent years, display apparatus typically having ascreen size in the 40-inch class have been becoming popular. However,some of the display apparatus with a screen size in the 40-inch classhave a backlight section inverter with a power consumption of about 200W. If a backlight section inverter has a large power consumption asdescribed above, the magnitude of a power loss incurred in every powerconversion process also increases to a relatively large value if thedisplay apparatus employing the backlight section inverter is designedinto the conventional configuration. Thus, the conventionalconfiguration raises a large number of problems.

The idea of the present invention is adopted as a technology for solvingthe problems. By applying the present invention to liquid crystaldisplay apparatus as described above, a power-loss reduction effect canbe obtained and, the larger the size of the display screen, the greaterthe power-loss reduction effect. Thus, in keeping up with futureenvironment changes such as increases in display-screen size, theimportance of the present invention is considered to increase.

Next, the configurations of power-supply apparatus each serving as amodified version of the first embodiment are explained.

FIG. 4 is a block diagram showing another simplified configuration ofthe power-supply apparatus 11 according to the first embodiment.

In the power-supply apparatus 11, a PFC (Power Factor Correction)converter circuit 7 is employed as a substitute for therectification/smoothing section 1 shown in FIG. 1. That is to say, asone of countermeasures for eliminating power-supply harmonicdistortions, for example, a converter for improving the power factor isprovided at a stage preceding the main power-supply circuit. Thus, thepower-supply apparatus 11 includes the PFC converter circuit 7 at astage in front of the main power-supply circuit 2 as well as in front ofthe inverter circuit 4.

A typical configuration of the PFC converter circuit 7 is shown in FIG.5.

The PFC converter circuit 7 shown in the figure is a voltage-boostingconverter adopting the PWM control method. The PFC converter circuit 7operates at a power factor approaching 1 so as to stabilize the directcurrent input voltage Ei.

First of all, as shown in the figure, the alternative current inputvoltage VAC generated by the commercial alternative current power-supplyAC is supplied to the input terminals of a bridge rectification circuitDi employed in the PFC converter circuit 7. An output capacitor Co isconnected between the plus and minus lines of the bridge rectificationcircuit Di in parallel to the bridge. A rectified output generated bythe bridge rectification circuit Di is supplied to the output capacitorCo. Thus, a direct current input voltage Ei is obtained between theterminals of the output capacitor Co as shown in the figure.

The direct current input voltage Ei is supplied to the main power-supplycircuit 2 and the inverter circuit 4 as shown in FIG. 4.

The configuration for improving the power factor as shown in the figureincludes an inductor L, a high-speed recovery diode D and a switchingcircuit Q3.

The inductor L and the high-speed recovery diode D are connected as aseries circuit between the plus output terminal of the bridgerectification circuit Di and the plus terminal of the output capacitorCo.

A MOS-FET is selected as the switching device Q3. As shown in thefigure, the switching device Q3 is connected between the minus line ofthe bridge rectification circuit Di and a point of connection betweenthe inductor L and the high-speed recovery diode D.

The switching device Q3 is driven by a driving control circuit not shownin the figure.

The driving control circuit typically executes PWM control based on thealternative current input voltage VAC and differentials of the directcurrent input voltage Ei to change the on duration of the switchingdevice Q3. The on duration of the switching device Q3 is referred to asthe duty of the switching device Q3. As a result of the control, thewaveform of an alternative current input current flowing to the bridgerectification circuit Di matches the waveform of the alternative currentinput voltage VAC. That is to say, the power factor is improved in orderto approach 1.

In addition, in this case, the duty (or the on duration) of theswitching device Q3 changes also in accordance with differentials of thedirect current input voltage Ei. Thus, variations in direct currentinput voltage Ei are also suppressed. That is to say, the direct currentinput voltage Ei is stabilized thereby.

Also in this modified version of the power-supply apparatus 11, theinverter circuit 4 generates an alternative current voltage for drivingthe backlight section not through the main power-supply circuit 2. Thus,the power loss incurred in a process to generate an alternative currentvoltage for driving the backlight section can be reduced to a quantitysmaller than that of the conventional configuration. That is to say, inthis case, in comparison with the conventional configuration shown inFIG. 7, the power loss is small even if a circuit equivalent to the PFCconverter circuit 7 is employed in the conventional configuration.

In addition, in the case of the configuration employing the PFCconverter circuit 7, the direct current input voltage Ei supplied to themain power-supply circuit 2 and the inverter circuit 4 is stabilized.Thus, the inverter circuit 4 can be designed on the assumption that astable direct current input voltage is supplied to the inverter circuit4. As a result, since the design of the inverter circuit 4 is simpler,the inverter circuit 4 is very advantageous from the practical point ofview if its combination with the configuration for improving the powerfactor is to be taken into consideration.

FIG. 6 is a circuit diagram showing a simplified configuration of apower-supply apparatus 12 according to a second embodiment of thepresent invention. It is to be noted that sections shown in FIG. 6 assections identical with their respective counterparts shown in FIG. 1are denoted by the same reference numerals as the counterparts, andtheir explanations are not repeated.

The power-supply apparatus 12 shown in FIG. 6 is also used as apower-supply section of a liquid crystal display apparatus 21. That isto say, the power-supply apparatus 12 supplies driving power to the load3 and a backlight section 15 as shown in the figure.

In addition, in this case, the backlight section 15 of the liquidcrystal display apparatus 21 employs LEDs. The power-supply apparatus 12supplies direct current driving currents to the backlight section 15.

A configuration for supplying direct current driving currents to thebacklight section 15 includes a plurality of DC-DC converters 9a, 9b and9c.

In this case, the backlight section 15 has a plurality of seriescircuits each including a predetermined plurality of LEDs connected toeach other in series. As the series circuits for supplying a directcurrent to the series circuits including a predetermined plurality ofLEDs receptively, a plurality of the DC-DC converters 9a, 9b and 9c areprovided.

As shown in the figure, a direct current input voltage generated by therectification/smoothing section 1 is supplied to the primary sides ofthe DC-DC converters 9a, 9b and 9c, and the primary sides of the DC-DCconverters 9a, 9b and 9c are not isolated from the commercialalternative current power-supply AC. That is to say, much like theinverter circuit 4 employed in the configuration shown in FIG. 1, theDC-DC converters 9a, 9b and 9c are connected to therectification/smoothing section 1 in parallel to the main power-supplycircuit 2.

In addition, almost in the same way as the configuration of the mainpower-supply circuit 2, each of the DC-DC converters 9a, 9b and 9cincludes an isolation transformer for isolating the commercialalternative current power-supply side and the load side from each other.On the primary side of the isolation transformer, a switching device anda driving circuit for driving the switching device are provided and, onthe secondary side of the isolation transformer, arectification/smoothing circuit is provided to form the configuration ofa switching converter. That is to say, the direct current input voltagesupplied to the primary side is subjected to a DC-DC power conversionprocess to generate another direct current voltage on the secondaryside.

In addition, each of the DC-DC converters 9a, 9b and 9c also includes acontrol system for stabilizing a direct current to be supplied to theseries circuit composed of a predetermined plurality of LEDs. Such acurrent stabilization control system typically has a detection circuit4d, 4e and 4f for detecting the level of a current flowing through theseries circuit of the LEDs and a feedback circuit 4g, 4h and 4i forfeeding back a voltage detected by the detection circuit to the primaryside through the isolation provided by the isolation transformer. Inthis configuration, in accordance with the voltage detected by thedetection circuit and supplied by the feedback circuit, the switchingfrequency of a driving signal supplied by the driving circuit to theswitching device is changed under control executed by the drivingcircuit.

Also in the configuration of the power-supply apparatus 12 according tothe second embodiment described above, the power conversion means forgenerating a power-supply voltage for driving the backlight sectionemployed in the liquid crystal display apparatus is not provided at astage behind the main power-supply circuit 2, but provided in parallelto the main power-supply circuit 2 at a stage behind therectification/smoothing section 1. That is to say, also in theconfiguration of the power-supply apparatus 12 according to the secondembodiment, the power-supply voltage for driving the backlight sectioncan be obtained by carrying out a power conversion process only once ineach of the DC-DC converters 9a, 9b and 9c. Thus, the power lossincurred by the power-supply apparatus can be reduced to a value smallerthan that of the conventional configuration shown in FIG. 8.

In addition, also in the power-supply apparatus 12 according to thesecond embodiment, a direct current voltage for driving the backlightsection 15 can be obtained not through the main power-supply circuit 2.Thus, it is no longer necessary to cope with an increased power consumedby the main power-supply circuit 2 as a power caused by an increasedsize of the display apparatus.

On top of that, the DC-DC converters 9a, 9b and 9c are connected to therectification/smoothing section 1 in parallel to the main power-supplycircuit 2 as described above. Thus, also in the power-supply apparatus12 according to the second embodiment, the larger the power consumptionfor driving the backlight section, the greater the obtained power-lossreduction effect in comparison with the configuration shown in FIG. 8 asthe conventional configuration.

In addition, also in this case, the main power-supply circuit 2 nolonger needs to supply power to the backlight section 15. Thus, thepower-supply specifications of the main power-supply circuit 2 need torely only on conditions of the load 3.

It is to be noted that, in the second embodiment, a plurality of DC-DCconverters 9 connected in parallel is provided. This is because, if aplurality of LEDs connected to each other to form a series circuit is tobe driven by only one DC-DC converter 9, the size of the DC-DC converter9 will increase as is the case with the configuration shown in FIG. 8 asa configuration in which, if a plurality of LEDs connected to each otherto form a series circuit is to be driven by only one chopper regulator109, the size of the chopper regulator 109 will increase.

In addition, particularly in this case, a plurality of DC-DC converters9 is connected to form a parallel circuit. Thus, in comparison with aconfiguration employing only one DC-DC converter 9, for each of theDC-DC converters 9, the core size and endurance voltage of the isolationtransformer can be reduced to result in a compact device. As a result,the increase of the total size of the DC-DC converters 9a, 9b and 9c canbe made infinitesimal.

On top of that, in the same way as the modified versions describedearlier by referring to FIGS. 4 and 5, the power-supply apparatus 12according to the second embodiment may employ a PFC converter circuit 7as a substitute for the rectification/smoothing section 1 even thoughthe power-supply apparatus 12 employing a PFC converter circuit 7 is notshown in any of the figures.

The second embodiment implements a typical configuration in which aplurality of LEDs is provided to form series connection circuits eachassociated with a DC-DC converter 9. In place of such a configuration,for series circuits each including a plurality of LEDs, only one DC-DCconverter can also be connected to the rectification/smoothing section 1in parallel to the main power-supply circuit 2. In this case, aplurality of isolation transformers each associated with one seriescircuit is provided in parallel inside the DC-DC converter 9 to form aplurality of DC-voltage generation systems. On the secondary side ofeach of the isolation transformers in the direct current-voltagegeneration systems, a direct current voltage is generated for the seriescircuit associated with the isolation transformer.

As another alternative, only one DC-DC converter can also be connectedto the rectification/smoothing section 1 in parallel to the mainpower-supply circuit 2, but a plurality of isolation transformers eachassociated with one series circuit is provided to form a series circuitinside the DC-DC converter 9 to form a plurality of directcurrent-voltage generation systems on the secondary side.

In the case of a DC-DC converter 9 including a plurality of isolationtransformers as described above, on the secondary side of each of thetransformers, the level of a current flowing through a series circuit ofLEDs is typically detected and, in accordance with the result of thedetection, the voltage generated on the secondary side is stabilized. Ifsuch a configuration is adopted, much like a plurality of DC-DCconverters 9 forming a parallel circuit, a direct current flowingthrough every series circuit of LEDs can be stabilized.

It is to be noted that, in accordance with the embodiments explained sofar, the power-supply apparatus provided by the present inventionfunctions as a power-supply section of a liquid crystal displayapparatus. In these embodiments, the inverter circuit 4 or the DC-DCconverter 9 generates respectively an alternative current voltage ordirect current voltage for driving the backlight section. However, thepresent invention can also be applied to a wide range of configurationsin which, for example, a second power conversion means generates analternative current or direct current power-supply voltage for driving aload other than the backlight section.

In addition, in the embodiments, a transformer employed in the invertercircuit 4 or the DC-DC converter 9 can be an electromagnetic transformeror a piezo-electric transformer.

The invention claimed is:
 1. A display apparatus having a backlightsection and a load other than said backlight section, said displayapparatus comprising: an input-voltage generation section for generatinga direct current input voltage from an alternating current; a firstpower conversion section for receiving said direct current inputvoltage, and for carrying out a DC-DC power conversion process on thedirect current input voltage to generate a direct current power-supplyvoltage to be supplied to said load; a second power conversion sectionconnected in parallel with said first power conversion section, andincluding: a primary side for receiving said direct current inputvoltage, and including: a pair of series connected switches forswitching the direct current input voltage to generate an alternatingcurrent input voltage being supplied to a primary winding of at leastone transformer, and a driving circuit for driving said pair of seriesconnected switches, a secondary side, isolated from said primary side bysaid at least one transformer, for generating, from an alternatingcurrent output voltage supplied by a secondary winding of the at leastone transformer, a power-supply voltage to be supplied to a plurality ofparallel connected backlights of said backlight section, a voltagedetection circuit connected in series with only one of the plurality ofparallel connected backlights for detecting a voltage supplied to thatbacklight, and a feedback section for receiving the detected voltagefrom said voltage detection circuit, for rectifying the detectedvoltage, and for supplying the rectified voltage to said driving circuitof said primary side of said second power conversion section, saiddriving circuit controlling the quantity of light emitted by theplurality of parallel connected backlights to a constant value based onthe rectified voltage; and a display section for displaying a pictureusing said backlight section.
 2. A display apparatus according to claim1, wherein the at least one transformer includes a plurality oftransformers each associated with a respective portion of the pluralityof parallel connected backlights of said backlight section.
 3. A displayapparatus according to claim 1, wherein a plurality of parallelconnected fluorescent tubes is are employed as the plurality of parallelconnected backlights of said backlight section, and said secondary sideof said second power conversion section generates the power-supplyvoltage to be supplied to each of said parallel connected fluorescenttubes.
 4. A display apparatus according to claim 1, wherein saidinput-voltage generation section includes a rectification/smoothingcircuit having a plurality of diodes for rectifying the alternatingcurrent, and a capacitor for smoothing a rectified output of saidplurality of diodes, and said input-voltage generation section generatessaid direct current input voltage as a voltage appearing betweenterminals of said capacitor.
 5. A display apparatus according to claim1, wherein said input-voltage generation section includes a power-factorimprovement converter for generating a stabilized direct current outputvoltage as the direct current input voltage.
 6. A display apparatusaccording to claim 1, wherein said feedback section isolates the primaryand secondary sides of the second power conversion section from eachother.
 7. A display apparatus having a backlight section and a loadother than the backlight section, the display apparatus comprising: aninput-voltage generation section which generates a direct current inputvoltage from an alternating current; a first power conversion sectionwhich receives the direct current input voltage, and carries out a DC-DCpower conversion process on the direct current input voltage to generatea direct current power-supply voltage to be supplied to the load; asecond power conversion section connected in parallel with the firstpower conversion section, and including: a primary side which receivesthe direct current input voltage, and including: a pair of seriesconnected switches which switches the direct current input voltage togenerate an alternating current input voltage being supplied to aprimary winding of at least one transformer, and a driving circuit whichdrives the pair of series connected switches, a secondary side, isolatedfrom the primary side by the at least one transformer, which generates,from an alternating current output voltage supplied by a secondarywinding of the at least one transformer, a power-supply voltage to besupplied to a plurality of parallel connected backlights of thebacklight section, a voltage detection circuit connected in series withonly one of the plurality of parallel connected backlights for detectinga voltage supplied to that backlight, and a feedback section whichreceives the detected voltage from the voltage detection circuit,rectifies the detected voltage, and supplies the rectified voltage tothe driving circuit of the primary side of the second power conversionsection, the driving circuit controlling the quantity of light emittedby the plurality of parallel connected backlights to a constant valuebased on the rectified voltage; and a display section which displays apicture using the backlight section.
 8. A display apparatus according toclaim 7, wherein the at least one transformer includes a plurality oftransformers each associated with a respective portion of the pluralityof parallel connected backlights of the backlight section.
 9. A displayapparatus according to claim 7, wherein a plurality of parallelconnected fluorescent tubes is employed as the plurality of parallelconnected backlights of the backlight section.
 10. A display apparatusaccording to claim 7, wherein the input-voltage generation sectionincludes a rectification/smoothing circuit having a plurality of diodeswhich rectify the alternating current, and a capacitor which smoothessmooths a rectified output of the plurality of diodes, and theinput-voltage generation section generates the direct current inputvoltage as a voltage appearing between terminals of the capacitor.
 11. Adisplay apparatus according to claim 7, wherein the input-voltagegeneration section includes a power-factor improvement converter whichgenerates a stabilized direct current output voltage as the directcurrent input voltage.
 12. A display apparatus according to claim 7,wherein the feedback section isolates the primary and secondary sides ofthe second power conversion section from each other.
 13. A displayapparatus having a backlight section and a load other than the backlightsection, the display apparatus comprising: an input-voltage generationsection for generating a direct current input voltage from analternating current; a first power conversion section for generating adirect current power-supply voltage to be supplied to the load; a secondpower conversion section connected in parallel with the first powerconversion section, and including: a primary side for receiving thedirect current input voltage, and including: a pair of series connectedswitches for switching the direct current input voltage to generate analternating current input voltage being supplied to a primary winding ofat least one transformer, a driving circuit for driving the pair ofseries connected switches, a secondary side, isolated from the primaryside by the at least one transformer, for generating, from analternating current output voltage supplied by a secondary winding ofthe at least one transformer, a power-supply voltage to be supplied to aplurality of parallel connected backlights of the backlight section, avoltage detection circuit connected in series with one of the pluralityof parallel connected backlights for detecting a voltage supplied tothat backlight, a feedback section for receiving the detected voltagefrom the voltage detection circuit, for adjusting the detected voltageto generate an adjusted optical signal voltage, and for supplying theadjusted optical signal voltage to the driving circuit of the primaryside of the second power conversion section, and the driving circuitcontrolling the quantity of light emitted by the plurality of parallelconnected backlights to a constant value based on the adjusted opticalsignal voltage; and a display section for displaying a picture using thebacklight section.
 14. A display apparatus according to claim 13,wherein the at least one transformer is a plurality of transformers eachassociated with a respective portion of the plurality of parallelconnected backlights of the backlight section.
 15. A display apparatusaccording to claim 14, wherein the plurality of transformers areparallel connected with respect to one another.
 16. A display apparatusaccording to claim 13, wherein the at least one transformer includes aplurality of secondary windings each associated with a respectiveportion of the plurality of parallel connected backlights of thebacklight section.
 17. A display apparatus according to claim 13,wherein a plurality of parallel connected fluorescent tubes are employedas the plurality of parallel connected backlights of the backlightsection, and the secondary side of the second power conversion sectiongenerates the power-supply voltage to be supplied to each of theparallel connected fluorescent tubes.
 18. A display apparatus accordingto claim 13, wherein the input-voltage generation section includes apower-factor improvement converter for generating a stabilized directcurrent output voltage as the direct current input voltage.
 19. Adisplay apparatus according to claim 13, wherein a junction locatedbetween the pair of series connected switches is connected to a firstterminal of the primary winding of the at least one transformer, and aterminal of the pair of series connected switches is connected through acapacitor to a second terminal of the primary winding of the at leastone transformer.
 20. A display apparatus according to claim 13, whereinthe junction point between the switches is connected to the primarywinding, and wherein one terminal of the switches is electricallyconnected to the primary winding by a capacitor.
 21. A power-supplyapparatus for generating a direct current power-supply voltage having abacklight section and a load other than the backlight section, thepower-supply apparatus comprising: an input-voltage generation sectionfor generating a direct current input voltage from an alternatingcurrent; a first power conversion section for generating a directcurrent power-supply voltage to be supplied to the load; a second powerconversion section connected in parallel with the first power conversionsection, and including: a first power-supply side for receiving thedirect current input voltage, and including: a pair of series connectedswitches for switching the direct current input voltage to generate analternating current input voltage being supplied to a first winding ofat least one transformer, a driving circuit for driving the pair ofseries connected switches, a second power-supply side, for generating,from an alternating current output voltage supplied by a second windingof the at least one transformer, a power-supply voltage to be suppliedto a plurality of parallel connected the backlight section, a voltagedetection circuit connected in series with one of the plurality ofparallel connected the backlight section for detecting a voltagesupplied to the backlight section, a feedback section for receiving thedetected voltage from the voltage detection circuit, for adjusting thedetected voltage to generate an adjusted optical signal voltage, and forsupplying the adjusted optical signal voltage to the driving circuit ofthe first power-supply side of the second power conversion section, andthe driving circuit controlling the quantity of light emitted by theplurality of parallel connected backlights to a constant value based onthe adjusted optical signal voltage; and the backlight section driving alight source as the load.
 22. A power-supply apparatus of claim 21,wherein the at least one transformer is a plurality of parallelconnected transformers.